This invention relates to a process for recovering tungsten and molybdenum from sulfur bearing material without polluting the atmosphere with gases such as sulfur dioxide and without production of sludge.
In the manufacture of tungsten from ores such as scheelite, wolframite, or scrap sources, the ores or scrap are processed by one of the four following conventional methods: acid decomposition, sodium carbonate digestion, fusion with sodium carbonate, or caustic digestion. These processes produce the desired sodium tungstate with smaller amounts of sodium molybdate as the major impurity. The molybdenum is removed by the addition of the sulfide or sulfide producing compound. In general, the process consists of adding sodium hydrosulfide, (NaSH), to the digested ore solution, adjusting the pH to approximately 3.0 to 4.0, using a mineral acid to precipitate the Mo, minor amounts of W, an appreciable amount of free sulfur, and other impurities such as Fe, As, and Sb which can be present. The resulting slurry is filtered to remove the precipitate, leaving a purified sodium tungstate solution. The precipitate contains appreciable amounts of both molybdenum and tungsten, and heretofore has not been upgraded. The results in loss of tungsten and molybdenum values and attendant storage problems.
Existing methods for recovering tungsten and molybdenum from sulfides and free sulfur are (1) roasting or air oxidizing, (2) oxidizing with nitric acid, and (3) fusion with sodium hydroxide or carbonate. The disadvantage of roasting in air is the formation of sulfur dioxide which is an air pollution problem. Nitric acid is both too expensive and produces free sulfur. Sodium hydroxide or carbonate fusion of the sulfides of tungsten and molybdenum results in the formation of sodium thiotungstates and sodium thiomolybdates respectively and sodium sulfide. Thus, sulfur is not removed from tungsten and molybdenum in this process.
It is believed, therefore, that a process which enables efficient removal of tungsten and molybdenum in a usable form from sulfur bearing material without disposal and air pollution problems of sulfur is an advancement in the art.